“Buji ni kaeru” is what people shout in Japan as the train starts
moving when someone is leaving: “Come back healthy!” or “Good luck on
your journey!”

And while you are still waving, you have already implored luck for the
person departing with one of these words. “Kaeru” (pronounced ka-eru) is
the word in question. Once you have spoken it, frogs prick up their
ears. Or rather, the ubiquitous spirit of frogs, which now endeavor to
do their best.

That is because they feel involved, because here the word “kaeru” means
not only “come back” or “turn around,” but also “frog.” This has led to a
close relationship emerging in Japan between lucky frogs and people.
And if people care for each other, they occasionally, for example at New
Year or as a farewell, exchange small frog figures, saying as they do
so “Fuku o mukaeru!,” which means “May you encounter luck!”